Several different types of silicon oxide are utilized in the fabrication of semiconductor devices. Two of the most common types are referred to as "thermal oxide" and "BPSG." Thermal oxide is typically composed of pure silicon dioxide and is utilized when an insulating layer is required. For example, thin "gate" layers of thermal silicon oxide are often utilized to separate conducting layers from each other. Thermal oxide layers on semiconductor wafers are usually prepared by high temperature oxidation of surface silicon on a polysilicon wafer. BPSG layers are comprised of silicon oxide which has been doped with boron and phosphorus. These layers serve the purpose of "gettering" alkali metal ion contaminants which could otherwise migrate into underlying layers and adversely affect electrical properties of the layer materials, causing device reliability degradation. BPSG layers are typically created by deposition of a suitable precursor material via atmospheric chemical vapor deposition ("CVD") followed by thermal treatment to density and planarize the layer. PSG layers serve the same gettering functions as BPSG layers but lack boron.
Various prior art formulations, as discussed herebelow, for etching various types of silicon oxide compounds typically etch BPSG layers and PSG layers more rapidly than thermal oxide layers, which can sometimes be a limiting factor in the fabrication of semiconductor devices.
Aqueous solutions of hydrofluoric acid ranging from very dilute solutions (&lt;1% by weight) to concentrated solutions (up to 49% by weight). The disadvantage of these solutions, in addition to lack of selectivity, is extremely high etch rates and metal corrosivity, making them suitable only for bulk etching of silicon oxide in the absence of exposed metal.
Nonaqueous solutions of hydrogen fluoride, Gajda, U.S. Pat. No. 4,230,523, discloses an etchant solution of hydrogen fluoride in an organic solvent such as glycerin or another polyhydric alcohol. The solution is utilized to etch silicon dioxide without etching silicon. Watanabe et al., U.S. Pat. No. 5,112,437 disclose solutions of hydrogen fluoride gas in methanol for removing silicon oxide films from semiconductor substrates.
Buffered oxide etchants ("BOEs") consisting of solutions of ammonium fluoride (or polyalkylammonium fluorides) and hydrofluoric acid. Ohmi et al., U.S. Pat. No. 5,277,835, disclose aqueous solutions of hydrogen fluoride and ammonium fluoride for surface cleaning of semiconductor wafers. Kinsbron et al., U.S. Pat. No. 4,343,677, utilize ammonium fluoride/hydrofluoric acid in a molar ratio of about 10:1 in water/ethylene glycol solvent for patterning silicon dioxide films. Roche, U.S. Pat. No. 4,921,572, discloses aqueous etchant solutions made from water, hydrogen fluoride, and ethylenediammonium difluoride or diethylenetriammonium trifluoride for etching of silicon dioxide.
Aqueous solutions of ammonium fluoride, Scardera and Roche, U.S. Pat. No. 4,871,422, disclose solutions comprised of ammonium fluoride and wetting agents in water for etching silicon dioxide layers.
Non aqueous solutions of ammonium fluoride, Bowden and Switalski, U.S. Pat. No. 5,320,709, disclose an etchant comprised of a solution of anhydrous ammonium fluoride in ethylene glycol for removing organometallic and organosilicon residues.
Related solutions, Ward et al., U.S. Pat. No. 5,571,447 disclose stripping compositions containing fluoboric acid, water, polyhydric alcohols other than ethylene glycol, and fluorine-containing compounds.
All of these prior art solutions have the limitation that they etch BPSG layers and PSG layers more rapidly than thermal oxide layers. The ratio of etch rates of BPSG to thermal oxide is approximately 4:1 for all of the prior art solutions. In other words, BPSG layers are etched about four times more rapidly than thermal oxide layers. Another way of stating this is that the etch rate "selectivity" of BPSG to thermal oxide is 4:1. To the inventors' knowledge there is no prior art or commercial etching solution that circumvents this limitation.